1. Field of the Invention
The present invention relates to the production and composition of superconductors that display resistance to environmental corrosion from materials such as water, CO.sub.2 and CO as well as chemical damage from photoresist and metal contact pad exposure. The improved chemical stability of these superconductors will enable more reliable processing methods to be exploited in the preparation of useful superconducting elements. The prospects for both thin film devices (Josephson junctions, SQUIDS, microwave devices, interconnects, bolometric sensors, etc.) as well as large scale applications for bulk materials (power transmission lines, superconducting magnets, motors, generators, maglev transportation, SMES, magnetic separators, etc.) are improved substantially with the availability of such stable superconductors.
2. Description of the Related Art
One of the major stumbling blocks that has plagued the practical utilization of high-T.sub.C superconductors has been the tendency of the cuprate compounds to degrade chemically when exposed to water, acids, CO.sub.2 and CO. (Zhou, 1992; Zhou, Appl. Phys. Lett. 1993; Barkatt, 1993; Rosamilia, 1987) Of the technologically important superconductors with transition temperatures above 77 K. (i.e. YBa.sub.2 Cu.sub.3 O.sub.7, Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10, Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8 and HgBa.sub.2 Ca.sub.2 Cu.sub.3 O.sub.9), YBa.sub.2 Cu.sub.3 O.sub.7 is the preferred material for thin film applications (Black, 1993; Bedekar, 1992; Narita, 1992). Unfortunately, of these cuprate materials, YBa.sub.2 Cu.sub.3 O.sub.7 displays the least corrosion resistance (Zhou, Appl. Phys. Lett. 1993).
Corrosion is a tremendous problem for thin film materials. Bulk superconductors are much less affected because the corrosion typically only attacks a surface layer and then is significantly hindered from reaching beyond that layer, although the entire material will eventually corrode. However, a thin film can not afford to have even a surface layer that is corroded as that will significantly affect its properties. Thus, it becomes imperative to either find a superconducting material that does not corrode or develop a way to protect the existing superconductors from environmental degradation.
There have been several methods suggested for providing protection against chemical damage for the high-Tc superconductor (Barkart, 1993), none of which has been shown to be entirely satisfactory. For superconducting wires, enclosure in a silver tube has been found to be successful in slowing corrosion. Unfortunately, this process does not completely halt degradation. Similar results are found for nitrogen ion implantation.
External protection methods such as coatings have also proven useful. Proposed and implemented coatings include fluoride, polymer, dielectric or fluorocarbon films.
In addition, a sol-gel process has been used to coat superconductors. Addition of TiO.sub.2 to such a film has led to improved corrosion resistance, but the critical temperature of the superconductor drops by 5 K. When TiO.sub.2 is added no the sol-gel coating, Ti.sup.4+ can replace Cu.sup.3+ as both are comparable in size (.+-.15%). It has been suggested that the reduction of Cu.sup.3+ is a primary factor in the corrosion process. The replacement of Ti.sup.4+ for Cu.sup.3+ then decreases the copper concentration near the surface and is thought to reduce the opportunity for corrosion.
However, it has been shown that YBa.sub.2 Cu.sub.3 O.sub.7-d (0&lt;d&lt;1) materials having intermediate oxygen contents corrode more slowly than do samples with either higher or lower oxygen contents (Zhou, Chem. Mater. 1993; Zhou, Solid State Comm. 1993). This behavior cannot be explained satisfactorily along the lines of copper oxidation levels. No other current theories adequately explain the corrosion mechanism. Without such a theory, any attempt at protecting the superconductors may not address the root of the problem.
Thus, all previous methods involved trying to protect superconducting material from the corrosive reactants through the formation of protective barriers. An alternate route is to find a superconducting material that is naturally resistant to corrosion. Such a corrosion resistant substance would have the advantage of avoiding extra steps involved in adding the protective element. The superconductor would also have a more flexible utility as no extra materials would be required that could hinder a particular use. Perhaps even more important is the fact that a more stable superconductor material will allow for easier processing of high-Tc products and devices. Currently, materials limitations have made it difficult to develop processing methods that can be exploited successfully for the preparation of useful superconductor thin film devices. Consequently, even short exposure to the atmosphere have been shown to cause significant amount of surface corrosion. This adverse chemical reactivity has been one of the most problematic issues that has slowed progress in the development of high-Tc thin film devices and sensors. The availability of stable superconductor surfaces will open new avenues for thin film processing and make it easier to establish electrical contact to the surface of a superconductor element. Moreover, the lifetime and reliability of superconductor elements will be effectively increased with the availability of these stable materials.